1. Field of the Invention
The present invention relates to a magneto-optical recording-reproducing method comprising the steps of (i) forming a record mark on a magneto-optical medium having a multi-layer structure; (ii) irradiating the magneto-optical medium with a light beam; (iii) displacing a domain wall of the record mark in a displacement layer without changing a record data in a memory layer by utilizing a temperature gradient in a temperature distribution caused by the irradiation of the light beam; and (iv) detecting a change in the polarization direction of reflected light of the light beam to reproduce the record mark. The present invention also relates to a magneto-optical recording reproducing apparatus used in such a method.
2. Related Background Art
As a rewritable high-density recording medium, a magneto-optical medium is known in which magnetic domains are formed in a magnetic thin layer thereof by thermal energy of a semiconductor layer to record information, and this information is read out by utilizing a magneto-optical effect. In recent years, there has been a strong demand for further enhancing of the recording density of this magneto-optical medium to provide a recording medium having a greater capacity.
The linear recording density of an optical disk, such as the magneto-optical medium, greatly depends on the laser wavelength and the numerical aperture of an objective lens of a reproducing optical system. More specifically, since the laser wavelength .lambda. and the numerical aperture NA of the objective lens of the reproducing optical system determine the diameter of a beam waist, the detectable range of the spatial frequency upon reproduction of a record mark is limited to about 2NA/.lambda.. Therefore, for actually achieving higher recording density with a conventional optical disk, it is necessary to shorten the laser wavelength .lambda. or enlarge the NA of the objective lens in the reproducing optical system. However, the improvements in the laser wavelength .lambda. and the numerical aperture NA of the objective lens are inherently limited.
Therefore, techniques have been developed where the composition and reading method of a recording medium are devised to improve the recording density.
For example, Japanese Patent Application Laid-Open No. 6-290496 proposes a signal-reproducing method and apparatus therefor, by which signals are recorded in a memory layer of a multi-layer film, having (i) a displacement layer and the memory layer magnetically connected to each other, and (ii) record marks of less than the optical diffraction limit of an optical system which are reproduced by displacing domain walls of record marks in the displacement layer without changing record data in the memory layer. This is done by utilizing a temperature gradient caused by irradiation of a light beam, magnetizing a part of the displacement layer in such a manner that the light beam-spotted region thereof becomes the same magnetization as the corresponding record mark, and detecting a change in the polarization direction of reflected light of the light beam. According to this method, the reproduction signals become rectangular as illustrated in FIG. 2E, so that record marks of frequency of less than the optical diffraction limit can be reproduced without decreasing the amplitude of reproduction signals depending on optical resolving power, whereby the magneto-optical medium and reproducing method can be greatly improved in recording density and transfer speed. FIG. 1 illustrates an example of the construction of the conventional magneto-optical recording-reproducing apparatus. In FIG. 1, reference numeral 1 indicates a magneto-optical disk comprising a substrate 2 composed of a glass or plastic material, a magneto-optical layer 3 formed on the substrate, and a protective layer 4 formed on the magneto-optical layer 3. The magneto-optical layer 3 has a multi-layer structure comprising a memory layer and a displacement layer and is capable of reproducing record marks of less than the optical diffraction limit of an optical system by displacing a domain wall of a record mark in the displacement layer without changing record data in the memory layer. This is done by utilizing a temperature gradient caused by irradiation of a light beam, enlarging magnetization within the light beam-spotted region of the displacement layer, and detecting a change in the polarization direction of reflected light of the light beam. The magneto-optical disk 1 is set to a spindle motor by a magnet chucking or the like, and is so constructed that it is rotatable on an axis of rotation.
Reference numerals 5 to 13 indicate individual parts that make up an optical head for irradiating the magneto-optical disk 1 with a laser beam and receiving information from reflected light. Specifically, the optical head comprises a condenser lens 6, an actuator 5 for driving the condenser lens 5, a semiconductor laser 7, a collimator lens 8, a beam splitter 9, a .lambda./2 plate 10, a polarized light beam splitter 11, photosensors 13 and condenser lenses 12 for the respective photosensors 13. Reference numeral 14 indicates a differential amplification circuit for differentially amplifying signals condensed and detected in the respective polarization directions. The laser beam emitted from the semiconductor laser 7 is projected onto the magneto-optical disk 1 through the collimator lens 8, the beam splitter 9 and the condenser lens 6. At this time, the condenser lens 6 is controlled so as to move in a focusing direction and a tracking direction while under the control of the actuator 5 in response to the detected signals from the photosensor 13 to successively focus the laser beam on the magneto-optical layer 3. This process is constructed so that it tracks along a guiding groove formed in the magneto-optical disk 1.
The laser beam reflected on the magneto-optical disk 1 is deflected by the beam splitter 9 to an optical path toward the polarized light beam splitter 11 and then travels through the .lambda./2 plate 10 and the polarized light beam splitter 11. Light beams split by the polarized light beam splitter 11 are condensed by the condenser lenses 12 on the respective photosensor 13 in accordance with the magnetization polarity of the magneto-optical layer 3. The outputs from the respective photosensors 13 are differentially amplified by the differential amplification circuit 14 in order to output magneto-optical reproduction signals.
A controller 16 receives information on the rotational speed of the magneto-optical disk 1, recording radius, recording sectors, and so forth, and outputs recording power, recording signals and the like to control an LD (laser diode) driver 15, a magnetic head driver 18 and the like. The LD driver 15 drives the semiconductor laser 7 and controls the recording power and reproduction power as desired.
Reference numeral 17 indicates a magnetic head for applying a modulation magnetic field to the laser irradiation site on the magneto-optical disk 1 upon the recording operation. The magnetic head is arranged in opposition to the condenser lens 6 with interposition of the magneto-optical disk 1. Upon recording, the semiconductor laser 7 applies recording laser power by irradiation of DC (direct current) light, under the control of the LD driver 15, and synchronously the magnetic head 17 produces magnetic fields of different polarities, under the control of the magnetic head driver 18, in accordance with the recording signals. The magnetic head 17 moves with the optical head in a radius direction of the magneto-optical disk 1, and applies a magnetic field successively upon recording onto the laser irradiation site of the magneto-optical layer 3, thereby recording information.
Guiding groove portions formed in the magneto-optical layer 3, between which respective land portions in a recording region have been formed, are preliminarily annealed at a high temperature by irradiating them with a laser beam of high power. This is done in order to modify the portions of the magneto-optical layer 3 corresponding to the guiding groove portions, so that domain walls of a record mark will not form a closed loop, or a closed magnetic domain. This treatment permits the displacement of the domain walls at a higher speed and the provision of stable reproduction signals.
The recording operation will be described by reference to FIGS. 2A to 2E, in which 2A illustrates recording signals, 2B recording power, 2C a modulation magnetic field, 2D a record mark sequence, and 2E reproduction signals. In recording of the recording signals as illustrated in FIG. 2A, the laser power of the semiconductor laser 7 is controlled with the start of the recording operation so as to give the prescribed level, as illustrated in FIG. 2B, and the modulation magnetic field, as illustrated in FIG. 2C, is applied in accordance with the recording signals, as illustrated in FIG. 2A, by the magnetic head 17. By these operations, the record mark sequence illustrated in FIG. 2D is formed in the course of the cooling of the magneto-optical layer 3. In the record mark sequence illustrated in FIG. 2D, hatching portions and dotted portions respectively indicate magnetic domains different in magnetizing direction from each other.
The reproducing operation will be described by reference to FIGS. 6A to 6D. Herein, a case is described where the magneto-optical layer 3 has a three-layer structure composed of a memory layer for controlling the storage of record marks, a displacement layer in which domain walls are displaced to directly contribute to reproduction signals, and a switching layer for switching the coupling state between the memory layer and the displacement layer.
FIG. 6A typically illustrates the reproduction state of magnetic domains. FIG. 6B illustrates the state of the magneto-optical layer 3 provided between the substrate 2 and the protective layer 4 of the magneto-optical disk 1. FIG. 6C illustrates a state diagram on the temperature of the magneto-optical layer 3. FIG. 6D illustrates reproduction signals. Upon reproduction, the displacement layer of the magneto-optical disk is heated by the irradiation of a light beam, as illustrated in FIG. 6A, up to an isothermal temperature Ts at which the domain wall in the displacement layer is displaced. In a temperature region lower than the isothermal temperature Ts, the switching layer, illustrated in FIG. 6B, is in a state coupled with the memory layer and the displacement layer by exchange-coupling. When the magneto-optical layer is heated to the isothermal temperature or higher by the irradiation of the light beam, the switching layer reaches its Curie temperature, so that the coupling between the displacement layer and the memory layer is broken. Therefore, a domain wall in the displacement layer is momentarily displaced to a position where the domain wall can remain energetically stable relative to the temperature gradient of the displacement layer, namely, to the maximum temperature point in the linear density direction of the temperature rise by the irradiation of the light beam so as to intersect a land. By this displacement, the magnetized state of most of the region covered with the light beam for reproduction becomes the same, so that reproduction signals of a substantially rectangular state, as illustrated in FIG. 6D, can be obtained even from minute record marks which have been unable to be reproduced by the ordinary reproduction principle using the light beam.
Accordingly, such a record mark sequence, as illustrated in FIG. 2D, is reproduced by a light beam whereby reproduction signals, illustrated in FIG. 6D, can be obtained. According to this method, the magnetized state of most of the memory layer and the displacement layer in the region covered with the light beam becomes the same, so that the reproduction signals become substantially rectangular, as illustrated in FIG. 6D. Therefore, record marks of less than the optical diffraction limit can be reproduced substantially without decreasing the amplitude of reproduction signals, whereby a magneto-optical medium and a reproducing method can be provided with great improvements in recording density and transfer speed.
However, the prior art described above has involved a problem that since the quality of the reproduction signals greatly depends on the reproduction power, the signal quality necessary for reproduction cannot be achieved as described below, when it is intended to reproduce information by the reproduction power preset as conventional, and so the reproduction of the information cannot be performed with precision.
In order to reproduce information stored in a magneto-optical layer having a multi-layer structure composed of a memory layer, a displacement layer and the like, on which the present invention is based, it is necessary to heat the magneto-optical layer up to a temperature Ts at which a domain wall in the displacement layer is displaced by irradiation of a light beam. At the same time as the domain wall of a record mark in the magneto-optical layer of the magneto-optical medium is heated to this temperature Ts, as a result of rotation of the magneto-optical disk, the domain wall is displaced to a position where the domain wall can remain stably. In this case, that is a position of the maximum temperature point in the linear density direction of the temperature rise by the irradiation of the light beam, where the domain wall intersects a land, whereby the magnetized state of most of the region covered with the light beam for reproduction becomes the same in a moment, so that reproduction signals undergoing steep polarity changes can be obtained. However, the temperature of the magneto-optical layer raised by the irradiation of the light beam varies according to the intensity of irradiation power of the light beam, the sensitivity to light, atmospheric temperature and transfer speed (linear speed) of the recording medium, and the quality of an optical system by which a light beam is formed.
The behavior of reproduction signals, in the case where the power of a light beam is changed, is described by reference to FIG. 7. FIG. 7 diagrammatically illustrates the dependence of the amplitude of reproduction signals and the amplitude of differential signals of the reproduction signals on reproduction power plotted on an abscissa when record marks of 0.15 .mu.m tone were recorded at a duty of 50%.
As apparent from FIG. 7, no reproduction signal appears in a first region of low reproduction power. In this region, a region of the temperature Ts is not formed at all on the medium, and so the phenomenon of domain wall displacement does not present itself. Therefore, the reproduction signals depend on the resolving power of the optical system like the reproduction of the conventional magneto-optical media. As a result, the record marks of 0.15 .mu.m tone cannot be reproduced due to the resolving power and intercode interference of the optical system.
In a second region in FIG. 7, a region of the temperature Ts starts to be gradually formed on the medium. However, since the region of the temperature Ts is insufficient, and a temperature gradient from the region of the temperature Ts to the maximum temperature point is gentle, the rate of domain wall displacement is also slow, and the polarity change of reproduction signals also becomes gentle, so that sufficient signal quality is not achieved. In this region, influence by the intraperipheral scattering and the like of the medium also presents itself.
In a third region in FIG. 7, a region of the temperature Ts is substantially sufficiently formed on the medium, and a temperature gradient from the region of the temperature Ts to the maximum temperature point becomes steep. Therefore, the rate of the domain wall displacement is also increased, and reproduction signals also come to show a steep change in polarity. Incidentally, in the third region, the reproduction signal carrier starts to decrease due to the increase in the reproduction power. This is mainly due to the fact that the temperature of a portion of the displacement layer contributing to a signal component within a light beam spot is raised by the increase of the reproduction power, and a Kerr rotational angle .theta..sub.k contributing to the amplitude of the reproduction signals is decreased with this temperature rise.
In a fourth region of higher reproduction power, data destruction in the memory layer starts, and so this region cannot be used as a reproduction region.
As apparent from the phenomenon described above, the reproduction waveform is changed to various states according to the intensity of the reproduction power. Therefore, due to the factors such as the sensitivity to light, atmospheric temperature and transfer speed (linear speed) of the recording medium, and the quality of an optical system by which a light beam is formed, it is very difficult to achieve the necessary signal quality by the reproduction power preliminarily specified.